This research proposal describes the development of an innovative gene therapy approach, Virus Directed Enzyme Prodrug Therapy (VDEPT), for the treatment of hepatic metastases of colorectal origin. This approach is based on the transfer of an artificial gene to colorectal cancer cells and selective sensitization of colorectal cancer cells, to a the nontoxic prodrug 5-fluorocytosine. The rationale for this proposal is based on numerous preclinical laboratory and animal studies which have demonstrated that chemosensitization of tumor cells to the prodrug 5-fluorocytosine results in the production of a high concentration of the chemotherapeutic agent 5-fluorouracil in solid tumors, and produces a dramatic tumoricidal response. For the gene therapy approach in humans, the chemosensitization gene will be delivered to tumor cells via a recombinant adenoviral-based vector. A key component of this research proposal is the development and characterization of novel adenoviral vectors to achieve the levels of transduction and transgene expression that will be clinically significant. The recombinant adenoviral vectors will contain the chemosensitization gene under the control of a tumor-specific promoter. The vectors will be characterized in vitro and optimized for transduction efficiency and specificity of transgene expression in target cell types. The recombinant adenoviral vectors will then undergo preclinical testing in animals to assess their safety and to demonstrate the feasibility of adenoviral-mediated gene transfer for treatment of solid tumors. Molecular, biochemical and pathological findings in the preclinical animal studies will define transduction efficiencies, specificity and level of transgene expression, prodrug conversion and cytotoxic effect. The results from preclinical in vitro and in vivo testing will contribute to the design of a Phase I clinical trial using the VDEPT approach in the treatment of patients with hepatic metastases of colorectal cancer. The primary goals of the clinical study are to l) establish the safety of the recombinant adenoviral vectors used in the gene therapy approach; and 2) to establish the feasibility of adenoviralmediated transfer of chemosensitization genes as a therapeutic modality for cancer patients. Results from the clinical study will also be used in early assessments of biological efficacy of the VDEPT gene therapy strategy. Thus, the results obtained from this clinical study will serve to define areas of future research which must be explored to achieve success in the new and promising field of gene therapy.